In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. After being dried, the paper is often run between drums which impart the desired smoothness. This process is referred to as calendering. The paper is then typically rolled into a jumbo roll at the reel, the end of the paper machine. Traditionally the quality of the paper is measured by a quality control system having sensors that are typically located just before the reel of the papermaking machine. By measuring the water content in the paper so late in the papermaking process, it is difficult to identify and correct the source of quality problems.
Papermaking machine press fabric, which is commonly referred to as “press felts” or “clothing,” plays a dual role in pressing operations. It supports and conveys the paper web of wet stock through the various operations and assists in paper web dewatering. It also acts as a transmission belt to drive other components of the press section. Similarly, dryer felts support the web in the dryer section of the papermaking machine. The typical press fabric run includes tensioning and positioning rolls and includes means to condition and dewater the felt to keep it permeable and open. A variety of mechanical and/or chemical conditioning treatments are used. Most systems utilize hydraulic energy (in the form of high- and low-pressure showers) as the primary means to loosen and flush out fines and fillers from the fabric structure. Following the showering treatments, the fabric is dewatered either by a suction box or a “wringer press.” If hydraulic energy by itself is insufficient, detergents and/or chemicals that serve as solvents and cleaning agents can be added.
Even when clothing is properly conditioned and maintained, the dewatering characteristics of the press felt and dryer felt change with time during normal operations of a papermaking machine. This is caused by normal wear on the fabric, especially if the wear is not uniform, and by the presence of excessive dirt which is not removed by conditioning. To compensation for fluctuations in dewatering characteristics, other machine variables can be manipulated to keep the paper web's quality within specification. Indeed, even new fabrics usually undergo a break-in period, during which time the papermaking machine cannot be operated at maximum speeds, before the fabric reaches an acceptable operating condition. Eventually, the fabric must be replaced due to excessive wear or compaction.
The principle technique for monitoring felts uses microwaves to measure the absolute level of water and to generate machine direction (MD) time trends and cross machine direction (CD) profiles of the water in the felts. The measurement is typically performed by a skilled operator who presses a sensor against the edge of a moving felt as he walks across the width of the felt. The recorded readings yield a profile of the moisture distribution.
The shortcomings to this monitoring process which relies on the manual dexterity and skills of an individual are obvious. Aside from the costs, inconsistencies, inherent physical limitations and dangers involved, no accurate “real time” data are generated for process control.
The art is in need of a precise and accurate automatic papermaking machine clothing monitoring system which can be employed to detect real time changes in the physical characteristics of the clothing which in turn can be employed to adjust machine operating parameters to compensate for the changes.